1. Field of the Invention
The present invention relates to an optical recording method and an optical recording apparatus for recording information by irradiating a medium with light.
2. Description of the Related Art
There have been proposed various bit-by-bit volumetric recordings with respect to a recording medium. For example, a 3D pit recording with respect to a transparent medium has been proposed (see, “Three-Dimensional Optical Data Storage in Vitreous Silica”, written by Hirao et al., Jpn. J. Appl. Phys. Vol. 37 (1998) pp. L1527-L1530 (hereinafter, referred to as Non-patent Document 1)). Further, mark formation on an organic medium with foam due to a nonlinear optical effect has been proposed (see, Japanese Patent Application Laid-open No. 2005-37658 (paragraph [0166]) (hereinafter, referred to as Patent Document 1)). Generally, in those volumetric recording methods, an ultrashort pulse is mainly used for recording, and a reaction process for forming a recording mark is considered to be completely the same as a reaction process of ablation. A hole formation by a laser ablation is considered to be caused by a combination of a photochemical reaction and a photothermal reaction (see, Laser handbook, Second edition, the Laser Society of Japan (hereinafter, referred to as Non-patent Document 2), p. 912).
For example, in a case where an ablation is caused with respect to a material using a CW (Continuous Wave) laser, high average light energy of several watts is required for causing a thermal decomposition that is a principle of the ablation, which causes a cost problem of a light source.
In contrast, in an ablation using an ultrashort pulse laser of a femtosecond to picosecond order, a photochemical reaction due to a high peak power and a short-time exposure is dominant, which allows the ablation with a lower average light energy.
A material irradiated with ultrashort-pulse laser light is photoexcited and causes dissociation, ionization, and the like due to the photochemical reaction. Generally, when the photochemical reaction occurs by the irradiation of the material with the ultrashort-pulse laser light, an absorption spectrum of the material is transiently changed, which causes light absorption to easily occur.
For example, when a metallic material is irradiated with the ultrashort pulse laser, an electron-system excitation due to a multiphoton absorption occurs, which generates a conduction electron, an ion, and a plasma (see, Non-patent Document 2, p. 926).
Also in a case of an organic molecule, when a savinyl-blue organic material is irradiated with a high-intensity ultrashort pulse, a change in electron state due to a light absorption and exciton-exciton relaxation occur, with the result that transient absorption change/increase is caused (see, “Dynamics study on femtosecond laser ablation of copper phthalocyanine solid”, doctor thesis, 2000, written by Yoichiro Hosokawa, Graduate School of Engineering, Osaka University (hereinafter, referred to as Non-patent Document 3), Chapter 3). An absorption time period in this case is several tens of picoseconds or more.